Ev, from LBDcaringspouses, sent me an email this morning about these two thought-provoking articles on memory in the Canadian newspaper National Post. The second article mentions Capgras Syndrome, which is common in Lewy Body Dementia.

In hospital etiquette, Alzheimer's patients pose a problem for the busy clinician passing them in the hallway. Is it rude not to say hello to someone who does not remember ever having met you?

Marcia Sokolowski, clinical ethicist at Baycrest Centre for Geriatric Care and the University of Toronto Joint Centre for Bioethics, once asked a patient her thoughts on this.

"I won't remember you, but say hi," the woman said. "I'll have a feeling about you."

Maybe she was just being polite, but her answer goes deeper than manners. It illustrates the multiplicity of memory. It hints at what science has slowly realized, that recognizing someone's face is not the same as remembering their name, or that you know them, or remembering a particular time you spent together. While each of these faculties may be damaged to the point of failure, others can more or less survive, and create the patient's vague "feeling" of familiarity.

Like pauses in Harold Pinter's plays, or silence in John Cage's music, the many kinds of forgetfulness stand in illuminating contrast to human memory. They show that remembering, like forgetting, is not always an all or nothing deal. It comes in parts, known as modules, such as memory for faces, skills, facts, or episodes in one's life.

Some of the more mysterious cognitive pathologies illustrate this aspect of memory, such as Capgras Syndrome, in which a person recognizes their loved ones' faces but feels no emotional familiarity, and so believes they have been replaced with impostors; or prosopagnosia, better known as "face-blindness," in which people cannot recognize faces, but can sometimes "figure them out" based on other cues, like hair or clothing or body shape.

But the clearest evidence for the diversity of memory has come from the physical brain itself, and from seeing what happens when the key brain structures involved in memory are diseased, injured, or surgically removed. These include the "stupid," seahorse-shaped hippocampus and the "emotional," almond-shaped amygdala, both deep in the middle of the head, behind the nostrils at the top of the spinal cord; and "the boss," the neocortex, behind the forehead.

Scientists have learned what kind of chemical changes must take place within and among brain cells in order to create, retain, recall and strengthen memories. They know where to look for the activity of memory in a healthy brain, and have a good idea of how and why specific forms of amnesia arise from specific kinds of brain damage.

"In that sense, we know what's going on," said Morris Moscovitch, a University of Toronto psychologist. "But the exact location where a memory is, I think, is something that we don't know and probably never will know."

He means that there is no such singular place, that memories are distributed in parts throughout the brain, and remembering is the process of gathering these parts together. This premise underlies his "multiple trace theory," a leading account of memory.

It begins with the observation that semantic memories, for general facts about the world, seem to last longer than episodic memories, for events in one's life. After showing this to be clearly true among Alzheimer's patients, Dr. Moscovitch and his colleague Lynn Nadel "found this disconcerting," he said.

One possible explanation was that facts (Paris is the capital of France) are called to mind more frequently than scenes (seeing the Mona Lisa for the first time), and so they are rehearsed and reinforced to a greater degree. Another is that plain facts are not terribly rich, content-wise, and are just more simply stored.

A few years ago, Dr. Moscovitch met the world's most famous amnesic, Henry Molaison, in a U.S. nursing home. He found him to be "a very simple person, a plain person."

In 1953, Mr. Molaison, known to the world as H.M. until his recent death, lost the ability to form new memories when two large chunks of his brain, including almost all his hippocampus, were surgically removed to cure his recurring seizures. But while he could no longer lay down new memories of his experiences, he did not seem to have lost his old ones. The hippocampus, therefore, seemed crucial to creating episodic memories, but they seemed to be stored elsewhere.

Also, his pre-existing semantic memories were largely preserved, and he remained generally intelligent, aware, and able to learn new skills. So memory for facts and skills also seemed to be happening outside the hippocampus.

The hippocampus is "stupid," Dr. Moscovitch says, because its role is to pay attention to anything that passes through consciousness, regardless of its relevance, truth, or possible usefulness. It does not care if you are experiencing something, dreaming it, or imagining it. It cannot predict what you will need to remember in future, and so its job is to make a record of everything, no matter what, but not to organize it.

It creates a linkage between itself and the neocortex, much like a reference card links to a book in a library. This link between two brain structures, this "ensemble of neurons," is the "trace" of "multiple trace theory."

The neocortex is a smarter brain region. It deals with complex and abstract thought, and gives order to the information flowing through the hippocampus. In evolution, it is the most recent of the brain's structures, and because of its role in giving an organizational structure to memory, Dr. Moscovitch calls it "the cross-examiner," or "the boss."

People with damage to this part of the brain often confabulate, or invent stories in a process psychologists call "honest lying."

"They will confabulate, tell stories about their past that are self-contradictory and patently false. Sometimes they're so bizarre that you don't need to know very much about the individual to know that the memory is impossible. But often it seems possible until you find the person contradicting himself," Dr. Moscovitch said.

In a recent paper, Dr. Moscovitch describes memory traces as "beads in a jar," meaning they are stored randomly, and independently of each other. In remembering, the neocortex gathers them together, compares their semantic content to see if it fits with other memories, and then "string[s] the... beads into different necklaces to be worn as befits the occasion."

When a memory is recalled, it returns to consciousness, so the hippocampus records it again, expanding and strengthening the original trace. Now, with multiple traces, the memory is strengthened and preserved.

"Every time we recover a memory, it's re-encoded in the hippocampus," Dr. Moscovitch said. "So the older the memory, the more chances you have of recovering it. The more chances you have of recovering them, the more re-encoding there will be, and the more traces of that memory there will be in the hippocampus. So there are âmultiple traces.'"

A peculiar consequence of this theory is that memory, while appearing to recapture the past, in fact can never happen the same way twice.

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